Minecraft won’t launch after changing username and logging in/out multiple times

There is a number of things you should rule out first:

• Are you able to login through http://account.mojang.com/login?

• Are you using the proper client from http://minecraft.net/download?

• If you were able to login with your old computer; make sure your current one doesn't have any firewall blocking internal / external connections.

• On the computer with the issue, do the following:

  : Go to this url, and substitute USER for your actual username and substitute PASS for your actual password.

  : It should output a Session ID (a bunch of numbers and letters), if not; and it returns Bad login or another exception; contact mojang.

What I have done that works, but only if you've opened the game once and logged in prior to this and saved the settings/profile, is to make sure no instance of the game not the loader is running. Then disable your internet, load the launcher and it should say 'play offline'. Press and play, once the game is loaded you should be able to safely reenable your internet.